a = float(input())
b = float(input())
c = float(input())

if a+b <= c or b+c <= a or a+c <= b:
    print("error")
else:
    p = (a+b+c)/2
    s = (p*(p-a)*(p-b)*(p-c)) ** 0.5
    print(format(s, ".3f"))
